The receptor-like kinase NIK1 targets FLS2/BAK1 immune complex and inversely modulates antiviral and antibacterial immunity

Among the factors that limit plant growth and productivities, pathogens cause major crop yield losses worldwide and represent serious threats to food security. Plants deploy various immune receptors to recognize pathogens and defend themselves. The first class of immune receptors is cell-surface associated pattern recognition receptors (PRRs), which are often represented by receptor-like kinases (RLKs) and receptor-like proteins (RLPs). PRRs recognize conserved structural motifs present in microbes, known as microbe- or pathogen-associated molecular patterns (MAMPs/PAMPs). Perception of PAMPs by PRRs activates PAMP-triggered immunity (PTI), which inhibits a broad spectrum of potential pathogens, including bacteria, viruses, fungi, and oomycetes. An antiviral defence mechanism uncovered recently, relies on host translation suppression mediated by a transmembrane immune receptor, NUCLEAR SHUTTLE PROTEIN-INTERACTING KINASE 1 (NIK1). NIK1 was first identified as virulence targets of the begomovirus nuclear shuttle proteins (NSPs). Loss of NIK1 function increases susceptibility to viral infection, whereas enhanced accumulation of NIK1 confers tolerance to begomovirus. The receptor-like kinase NIK1, was previously shown to positively regulate plant antiviral immunity, acts as an important negative regulator of antibacterial immunity. Plants that are nik1 null exhibit dwarfed morphology, enhanced disease resistance to the bacteria Pseudomonas syringae and enhanced bacterial flagellin–triggered PTI responses, which were restored by reintroduction of NIK1. In addition, NIK1 negatively regulates the formation of the FLS2/BAK1 complex and flg22-mediated responses. The proposed underlying mechanism for NIK1-mediated inverse modulation of antiviral and antibacterial immunity may allow bacteria and viruses to activate host immune responses against each other.